Resistors formed of ceramic materials of positive temperature coefficient of resistivity (PTC) are used in many applications as current limiting devices and as self-regulating heaters. When electrical current is directed through such materials, the materials tend to heat and display increasing resistivity so that current flow in the resistor is reduced whereby its rate of heat generation is decreased. When the rate of heat generation reaches equilibrium with the rate of heat dissipation from the resistor, the resistor temperature stabilizes and limits the resistor current to a predetermined level. The initial room temperature resistivity of a PTC material and the rate of change of resistivity with temperature are characteristic of the material, and the materials used in such resistors are commonly chosen to display a sharp anomalous increase in resistivity at a particular temperature, thereby to stabilize heating of the resistor at about that temperature while also reducing resistor current to a very low level at the stabilizing temperature.
PTC heaters have been in used for many years. Such heater offer several operating advantages over conventional resistance heating elements in the heating of fuels. They can be made in a flat shape and are formed, generally, of doped barium titinate ceramics which have a sharp positive temperature coefficient of resistance. The PTC heaters are designed such that below a critical temperature, the resistance of the ceramic remains at a low value and is essentially constant. When a particular temperature is reached, a crystalline phase change takes place in the ceramic and this change in crystal structure is accompanied by a sharp increase in the resistance at the crystalline grain boundaries. The result of this crystalline change is an increase in the heater resistance of several orders of magnitude over a very small temperature range. A barium titinate heater with a room temperature resistance of 3.0 ohms will increase to 1,000 ohms or more during the crystalline phase change. The temperature at which the crystalline phase change takes place can be adjusted in the PTC manufacturing process through the use of appropriate chemical dopents and can be varied between -50.degree. C. and 300.degree. C. When energized with a suitable voltage by applying current to the opposite sides of the PTC heater, the ceramic rapidly heats up to a predetermined operating temperature and then "locks in" at this temperature. This rapid heating is due to the initial low resistance of the PTC ceramic heater which results in an internal high power of the heater. The "lock in" is due to the abrupt increase in resistance which causes generated power to be reduced until it equals dissipated power. At this point, thermal equilibrium is achieved and the PTC heater self-regulates itself at that temperature.
In prior art devices where PTC resistors are used particularly as self-regulating heaters, various designs and assemblies have been employed in an effort to satisfy new applications when such PTC heaters could be employed. The requirements of such applications include maximizing heat transfer, maintaining the integrity of the PTC resistor from environmental effects, maintaining a substantially uniform distribution of heat to the medium to be heated, and delivering higher temperatures without degradation of the PTC heater. Equally important is that the PTC heater be a device having a structure which is rugged, compact, reliable and inexpensive and simple to form.
Examples of such prior art PTC resistors employed as heaters are disclosed in the following patents: U.S. Pat. No. 4,242,999 to Hoser which discloses heaters in the shape of a "pill"; U.S. Pat. No. 4,406,785 to Seiter which discloses a plurality of pill-like PTC heaters disposed in a ring-array; and U.S. Pat. No. 4,107,515 to Kulwicki which discloses a compact resistor device having a large number of passageways extending between opposite ends of the body.
While the foregoing prior art patents have provided improvements in the areas for which they were intended, there still exists a need to provide a compact PTC resistor which meets the application requirements disclosed above.
Accordingly, an object of the present invention is to provide a compact PTC resistor device which is particularly suitable or useful as a self-regulating heater that will maximize heat transfer while maintaining the integrity of the heater from environmental effects.
Another object of the present invention is to provide a compact PTC resistor device of the above desirable object which produces a substantially uniform distribution of heat to the medium to be heated while delivering higher temperature transfer without degradation of the device.